Head quarters. Done it on the first attempt. I have gold on the RPG and almost done SMAW. just need the AGR for the stinger.
BUT too your question on the triple....
On headquarters just cap the base then run and hide. once they start to reclaim the base jump out and fire at them. with any luck they wont be wearing a flack jacket. I have also had quad kill doing it this way.
groundwar domination on raid went to cap the B flag and fired a rpg at the van and got 4 kills and that's how I got it I know it was the van blowing up that got me the 4 kills but I'll take it I've heard 2gun kills and a final rpg kill works just as good but I've never tried it.
1st perks- lightweight & flak jacket (shoot enemy point blank)
2nd perks- scavenger (never run out of rockets ) & whatever you prefer.
3rd perks- tac mask & extreme conditioning (hate getting stunned and love running forever )
thats my personal opinion tho